There were studies into the radiation dose and shielding methods in FACET in May 2012.
Dose readings from approximately a week of beam running show doses of between 1.7 and 3.75 kilorad depending on location on the experiment tables.
1/2 inch Pb typically provides shielding factors between x 3.5 and x 7.
2" Pb, has a shielding factor between x 11 and x 57.
4" Pb shielding improves on the 2" shielding somewhere between x 4 and x 8.
6" Pb shielding essentially provides no additional benefit.
Thanks to Clive Field, Henry Tran and Santa Chatterji.